The present invention relates generally to amplifier systems, and more particularly to a feedback arrangement for correcting distortions in the amplifier system.
Generally, audio amplifiers, or the like, are designed to amplify an input signal and apply it to a load with a minimum amount of distortion. In systems using inductive loads, for example, a back EMF or voltage is generated by the load which is proportional to the inductance of the load times the rate of change of the current through the load. In general, most conventional amplifier systems used to drive inductive loads have been unable to do so without introducing distortion into the output signal. This distortion is due to the time varying back EMF change which either boosts or reduces the output signal, thus introducing the distortion.
One prior attempt to correct for distortion in inductive loads utilizes a fixed filter arrangement to provide a predistorted signal in the load. If the distortion characteristics of the load are known, the predistorted signal is shaped so as to interact with the load to produce an undistorted signal in a manner analogous to the concept of destructive interference. However, this approach produces acceptable results only over a limited frequency range. The approach is unacceptable when used with devices such as shaker table drivers, magnetostrictive devices, and loudspeakers, or the like, which operate over a wide frequency in harmonic ranges, and which are subject to varying loads over their operating ranges. Also, distortion in recording systems such as magnetic tape phonograph discs, or the like, may not be corrected by such prior art systems.
For example, many prior art systems have attempted to correct for distortion in amplifier systems incorporating loudspeakers. Many factors affect the distortion-free performance of the loudspeaker including speaker cone mass, air resistance, and self-resonances. Nonlinearities introduced by these speaker properties result in changes in the back EMF of the speaker which boosts or reduces the amplifier output signal, producing unwanted distortion.
Prior attempts to correct for distortion include various types of sensors in the speaker to detect speaker motion. Typical of these sensors are light beam systems, extra voice coils, and tapped voice coils. The sensors generate error signals by comparing signals proportional to the speaker motion and the amplifier input signal, and applying the error signals to the amplifier in a feedback arrangement. Typical of prior art systems utilizing feedback are U.S. Pat. No. 2,358,630, issued Sept. 19, 1944, for "Amplifier System"; and U.S. Pat. No. 3,656,831, issued Apr. 18, 1972, for "Feedback Amplifier."
Additionally, many prior art systems have incorporated transformers to either isolate the load from the amplifier or to provide voltage feedback to the amplifier in an attempt to correct for distortion caused by the amplifier and generally not by the load. Typical of such systems include U.S. Pat. No. 1,519,211, issued on Dec. 16, 1924 for "Loudspeaker Circuit"; U.S. Pat. No. 2,302,493, issued Nov. 17, 1942 for "Amplifying System"; U.S. Pat. No. 2,358,630, issued Sept. 19, 1944 for "Amplification System"; U.S. Pat. No. 2,383,867, issued Aug. 28, 1945 for "Power Output Amplifier Circuit"; U.S. Pat. No. 2,922,846, issued Jan. 26, 1960 for "Audio Amplifier System"; and U.S. Pat. No. 3,656,831, issued Apr. 18, 1972 for "Feedback Amplifier."
Generally, many of the prior art systems include many components which add to the system complexity, cost and reduced reliability. Also, those systems incorporating error-sensing resistors or the like generally have power losses associated with them due to the incorporation of the resistors.